glsl: Statically cast parameter exec_node to ir_variable.

Formal function parameters are always ir_variable objects, not an
arbitrary ir_instruction.  So there's no need to dynamically cast here.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/builtin_functions.cpp

@@ -2399,7 +2399,7 @@ builtin_builder::call(ir_function *f, ir_variable *ret, exec_list params)
    exec_list actual_params;
 
    foreach_list(node, &params) {
-      ir_variable *var = ((ir_instruction *) node)->as_variable();
+      ir_variable *var = (ir_variable *) node;
       actual_params.push_tail(var_ref(var));
    }